Photonic wire bonds

ABSTRACT

An optical arrangement includes a plurality of planar substrates with at least one planar integrated optical waveguide on each planar substrate. At least one optical waveguide structure has at least one end connected via an optical connecting structure to one of the planar integrated optical waveguides. The optical waveguide structure is positioned at least partly outside the integration plane for the planar integrated optical waveguide and a refractive index contrast between a core region and a cladding region of the optical waveguide structure is at least 0.01.

BACKGROUND

1. Technical Field

The invention relates to the field of micro-optics and nano-optics, andmore particularly relates to waveguide structures for opticalinterconnection of planar-integrated photonic systems (chip-chipconnections) and the connection of planar integrated photonic systems toglass fibers (fiber-chip connection). The invention furthermore relatesto a method and a device for producing the waveguide structures.

The field of integrated optics, in particular the area of siliconphotonics, meaning the integration of waveguide-based photoniccomponents on silicon or silicon-on-insulator (SOI) substrates, has beenthe subject of intensive research and development work for severalyears. This technology permits transferring mature CMOS (complementarymetal oxide semiconductor) processes, developed for mass production ofintegrated electronic circuits, to the integrated photonic.Nano-photonic systems with high complexity and a plurality offunctionalities can thus be integrated into the smallest possible spaceand produced on an industrial scale. The areas of application for suchsystems are primarily in the field of data transmission, as well as foroptical measuring technology and sensing technology. The economicpotential of silicon photonics is high and first products are alreadycommercially available.

However, the design and connection technology for integrated photonicsystems has increasingly proven to be an obstacle for furthercommercialization. Complex photonic systems are based on lateralsingle-mode, planar integrated waveguides. Waveguide structures arecalled “lateral single mode” for which in each polarization only thebasic mode can propagate. Lateral single-mode waveguides thus generallyhave two guided modes with different polarizations, for example called“quasi TE” and “quasi TM”. A low-loss optical connection of thesesystems is therefore only possible with waveguide structures whichpermit an efficient connection to the basic modes of the integratedwaveguide. For the optical connection of integrated photonic systems,standard single-mode fibers are generally used which are affixeddirectly to the chip, using a high number of manual operational steps.This leads to a correspondingly low integration density of opticalchip-to-chip connections and results in high packaging costs which, inpart, amount to more than 50% of the total costs for the system. In thefield of microelectronics, the development of reliable “wire bonding”techniques was a basic requirement for economic success. Methods with acomparable throughput and degree of automation have so far not beenavailable for the field of photonics.

For connecting integrated optical components, passive positioningmethods are primarily used for which a transmitter chip as well as areceiver chip must be positioned with corresponding devices andsufficient precision, relative to the waveguide. The precisionrequirements are for the most part determined by the cross-sectionalsurfaces of the waveguides. Thus, systems are often preferred for themass production of systems which are based on multi-modal waveguideswith large cross-sectional surfaces. In recent years, techniques havebeen developed which permit, for example, a multi-modal connection ofsurface-emitting laser sources (so-called vertical cavity surfaceemitting lasers (VCSEL)) on a transmitter chip with planar integratedphoto detectors on a receiver chip.

However, these methods cannot be used for connecting lateral single-modeintegrated optical waveguides. Waveguide diameters in the field ofsilicon photonics typically are noticeably below 1 μm. The resultingprecision requirements cannot be met satisfactorily with adjustmenttechniques only.

Further, over the last years, several photonic integration platformshave gained maturity, each of them having individual strengths andweaknesses. III-V-compound semiconductors have become the mainstay foroptical sources and amplifiers, whereas silicon photonics enablesefficient co-integrating of modulators and detectors together with WDMfunctions, and optical waveguides based on silicon nitride are used forhigh-performance passive devices. However, there is currently noflexible and cost-effective method that could combine these technologiesinto a joint module.

Single-mode integrated optical components are presently connected almostexclusively with the aid of single-mode fibers which are connecteddirectly to the end face of an integrated waveguide. In the process, thefibers and the waveguide facets must be positioned and attached withhigh precision, relative to each other. As a rule, this is achieved bymeasuring the optical coupling efficiency during the alignment and bymaximizing the positioning of the fiber tip, wherein this is alsoreferred to as “active alignment.” The mode field adaptation between thesingle-mode fiber and the integrated waveguide as a rule occurs bygiving the fiber tip a special form which ensures a focusing of theexiting light (so-called “lensed fibers”). In addition, integratedoptical tapers are often used which optimize the coupling efficiencybetween the focused mode field for the single-mode fiber and theintegrated waveguide.

This method was developed for individual fiber-chip connections whichare not scalable for a large number of optical connections and cantherefore no longer meet the requirements of a high-quality integratedphotonic system. The geometric dimensions of standard single-mode fibers(diameter approximately 125 μm), for example, limit the achievableintegration density. In addition, the high expenditure for an activeadjustment is no longer acceptable for the mass production of integratedoptical systems. The active adjustment with passive integrated opticalsystems is furthermore difficult since these systems do not contain aninherent light source which would allow detecting and optimizing thecoupling efficiency during the adjustment operation. Passive adjustmentmethods continue to suffer from poor reproducibility of the couplingefficiency. Furthermore problematic are the generally high opticallosses with chip-fiber couplings (50% are no rarity) which, above all,can be traced back to poor mode field adaptations between thesingle-mode fiber and the integrated waveguides. Since the integratedwaveguides generally are planar structures, a mode field adaptationfrequently is possible only in the substrate plane.

2. Prior Art

German patent document DE 10 2007 055530 A1 describes a method forlaser-beam processing of a work piece. Among other things, thespecification discloses a method for finding planar surfaces within thework piece to be processed. Three-dimensional structures cannot bedetected with this method and the production of waveguides as well astheir connection to pre-positioned structures is not mentioned therein.

German patent document DE 601 14 820 T2 describes use of multi-photoninduced photo structuring methods for producing three-dimensionaloptically functional structures in polymer or oligomer materials. Thedescription is concentrated on the lithography methods and the resistmaterials. The adaptation and connecting of the generated structures topre-positioned components is not mentioned.

German patent document DE 601 30 531 T2 describes production of opticalwaveguides of a light-hardening resin. The description is focused on thesequences of different illumination steps, which are used to define thecore region and the sheath region of the waveguide. The resins here arestructured along the propagation direction for a light beam. Thewaveguides thus consist of a series of, and in some parts straight,sections. Free form curves with variable waveguide cross sections cannotbe created with this method. Accordingly, the problem definition uponwhich the present invention is based cannot be solved this way.

German patent document DE 10 2007 038 642 A1 describes three-dimensionalstructuring of waveguides with variable cross-sectional geometries inpropagation direction by using multi-photon processes. A local increaseof the refractive index, induced by the radiation, is used for the lightguidance. The consequently achievable index contrast is low (typically0.005), so that realization of compact photonic wire bonds is notpossible with this method. The adaptation and/or the connection of thewaveguide structures, generated in this way, to pre-positioned opticalcomponents is not the subject matter of the specification.

International patent application publication WO 2009/021256 A1 describesa method for producing optical waveguides on polymer substrates. Thespecification concentrates on the lithographic method and the resistmaterials, wherein the three-dimensional structuring with the aid oftwo-photon absorption processes is also mentioned. The adaptation and/orthe connection to the waveguide structures, generated in this way, topre-positioned optical components is not part of the subject matter ofthe specification.

European patent document EP 0 689 067 A2 describes a method for theoptical structuring of connecting waveguides between pre-positionedcomponents. The light beam used for the structuring is radiated directlyfrom the ends of the waveguides to be connected into a non-linearoptical material. In regions of high optical intensity, meaning alongthe light rays and in particular at the crossing points of light rays,an optically induced polymerization reaction takes place, which leads toforming waveguide structures along the paths for radiating in light.These waveguide structures are oriented per definition on the opticalelements to be connected, but the waveguide geometries that can becreated with this method are strongly limited. In particular, it is notpossible to generate pre-computed and optimized free-form waveguides.The achievable index contrast is furthermore very low and the componentsto be connected must be positioned, relative to each other, with highaccuracy. The use of passive components is furthermore also made moredifficult in that these are frequently not transparent for single-photonprocesses at the lithographic wavelength. The use of multi-photonpolymerization processes in most cases fails because the requiredcapacities frequently cannot be transported in passive structures. Theproblem definition upon which the present invention is based cannot besolved with this method.

German patent document DE 19545 721 C 2 describes a method for producingoptical micro-components on fiber end surfaces and/or laser facet. Forthis, the position of the region where the optical micro-component is tobe generated is first detected with an imaging method. Based on the dataobtained, the optical micro-component is then generated with highrelative accuracy on the fiber end surface or the laser facet. The termoptical micro-component in this case refers to lenses or prisms. Theconnection and geometric adaptation of waveguides on pre-positionedoptical components is not mentioned and is not feasible with this methodsince the described imaging method does not permit the three-dimensionalposition detection. The described method thus cannot be used to solvethe problem defined for the present invention.

Schmid, G.; Leeb, W.; Langer G.; Schmidt, V & Houbertz, R., “Gbit/stransmission via two-photon absorption-inscribed optical waveguides onprinted circuit boards;” Electronic Letters, 2009, pp. 45, 219-221discloses the production and function demonstration of a multimodewaveguide produced in a volume of a resist material and/or a multi-corewaveguide which connects a VCSEL (vertical cavity surface emittinglaser) and a photodiode. With the integrated components to be connected,light is coupled in and coupled out via the surface of the substrate.The problem of connecting to a single-mode planar integrated waveguidedoes not arise with this method. The problem upon which the presentinvention is based thus cannot be solved with the above-describedmethod. The structure described therein furthermore has a very lowrefractive index (estimated at 0.005) which does not allow reaching thehigh integration density required for photonic wire bonds.

Schmidt, V; Kuna L.; Satzinger, V.; Houbertz, R.; Jakopic, G. & LeisingG.; “Application of two-photon 3D lithography for the fabrication ofembedded ORMOCER waveguides;” Porc. SPIE, Vol. 6476 discloses theproduction of waveguide structures with two-photon polymerization. Thewaveguides are based on a local increase in the refractive index whichis induced by the radiation. The waveguides connect VCSELs with theassociated photodiodes. The diameter of the waveguides is describes asmeasuring “tens of microns” and it may be assume that multimodalwaveguides are used here as well which is confirmed by the intensitydistribution shown in the publication. The described waveguides aretherefore in principle not suitable for connecting single-modeintegrated optical components. Accordingly, connecting of embeddedwaveguides to planar integrated lateral single-mode waveguide structuresis not taken into consideration. The low refractive index differencefurthermore results in extremely large structures (length of waveguide2-12 cm), which gives reason to assume correspondingly large radii forthe waveguide curvatures. The integration density necessary for photonicwire bonds cannot be achieved with this method. The specificationfurthermore discloses that the complete waveguide cross section isgenerated during a single writing passage. For this purpose, a telescopecomposed of cylindrical lenses is arranged in the beam path which allowsa corresponding adaptation of the shape of the focusing region in theresist material. From this it can be assumed that the spatial resolutionthat can be achieved with the aforementioned arrangement is in the rangeof 10 μm, which prevents an efficient optical connection of thegenerated structures to lateral single-mode, planar integratedwaveguides. The production method described in the publicationfurthermore contains a position detection of the optical components tobe connected with the aid of a so-called “machine vision system.” A CCDcamera is therefore used for the lateral position detection and isinstalled adjacent to the microscope objective. The accuracies which canbe achieved with this arrangement are limited and, at best, shouldamount to a few micrometers. This is sufficient for multimodalconnection waveguides. However, the connecting of single-mode connectionwaveguides to pre-positioned components is not possible with this systemfor lack of accuracy. In the axial direction, the machine-vision systemonly detects the position of the sample surface with the aid of aconfocal arrangement. A three-dimensional position detection ofcomponents embedded in the resist material is not intended and,accordingly, this arrangement cannot be used to generate waveguidestructures which are connected directly and with high precision toplanar integrated waveguide-based components. The measured insertionloss for waveguides produced in this way amounts to 7.8 dB, wherein suchhigh values cannot be tolerated when connecting nano-photonic systems.

Houbertz, R.; Satzinger, V.; Schmid V.; Leeb, W. & Langer, G.;“Optoelectronic printed circuit board: 3D structures written bytwo-photon absorption; Organic 3D Photonics Materials and Devices II,”SPIE Int. Soc. Optical Engineering, 2008, Proceedings Vol. 7053,B530-B530 is closely connected to the above-discussed publication. Itdescribes the production of waveguide structures for connectingpre-positioned components. A local increase of the refractive index,induced by a two-photon process, is used also in this case to definewaveguide structures. The resulting structures, however, have extremelylarge cross sections and are therefore multimodal and not suitable forsolving the object of the present invention. The waveguides furthermorehave a low index contrast and correspondingly large curvature radii andtherefore cannot meet the requirements for photonic wire bonds withrespect to the integration density. Again, the lateral position of thecomponents to be connected is detected with the aid of a camerainstalled adjacent to the microscope objective. The position detectionin axial direction is limited to the detection of the upper edges of theVCSEL and photo diode chips which are fixated perpendicular to thesurface of the component carrier. It can be assumed that the relativeposition accuracies which can be achieved with this arrangement are notsufficient for connecting single-mode planar integrated waveguideshaving cross sections of only a few micrometers. The generated waveguidestructures furthermore cannot be connected directly to the surface ofthe chips—the waveguides end (start) at a distance of approximately 10μm to the photodiode (the VCSEL). Technical reasons for this are notdisclosed and it has be assumed that the shadowing effects caused by thevertically mounted chips play a role. The above-described arrangementcannot be used for producing photonic wire bonds which can be connecteddirectly via corresponding connecting structures to planar integratedwaveguides.

Houbertz, R; Wolter, H.; Dannberg, P; Serbin, J. & Uhlig, S.; “Advancedpackaging materials for optical applications: bridging the gap betweennm-size structures and large-area panel processing;” Art. No. 612605,Photonics Packaging and Integration VI, 2006, pp. 6126, 12605-12605discusses inorganic-organic hybrid polymers (so-called ormoceres) withthe associated structuring methods based on two-photon polymerizationand their uses for the optoelectronics. Discussed as example, amongother things, is the production of optical components with two-photonpolymerization, wherein it is mentioned as an advantage that thesecomponents can be realized on substrates which already containpre-structured components such as VCSELs or micro-lenses. However, thepublication does not discuss the coupling of TPP structured waveguideswith integrated optical waveguides. The waveguides described therein aremultimodal and thus cannot be coupled without loss to single-mode planarintegrated waveguides. The problem defined for the invention thereforecannot be solved with the methods described in this publication.

SUMMARY OF THE INVENTION

An object of the invention is to provide a technology which permits alow-loss optical connection of lateral single-mode, planar integratedphotonic systems.

It is a further object of the invention to provide an optical waveguidestructure that can be freely designed in three dimensions. Such opticalwaveguide structures are also referred to herein as photonic wire bonds(PWB). Photonic wire bonds are intended to permit high integrationdensities while, simultaneously, allowing an economic production withlarge piece numbers.

The above and other objects are accomplished according to the inventionby the provision of an optical arrangement, which in one embodiment,includes: at least one planar substrate; at least one optical waveguideplanar integrated on the at least one planar substrate; opticalconnecting structure; and at least one optical waveguide structurehaving at least one end connected via the optical connecting structureto the at least one planar integrated optical waveguide, wherein the atleast one optical waveguide structure is positioned at least partlyoutside the integration plane for the at least one planar integratedoptical waveguide, and a refractive index contrast between a core regionand a cladding region of the at least one optical waveguide structure isat least 0.01.

Thus, the inventive optical waveguide structures (photonic wire bonds)consist of a material which can be processed with a high-resolution,three-dimensional structuring technique. The photonic wire bonds may beoptically connected via special connecting structures to a planarintegrated optical waveguide, for example, a lateral single-mode, planarintegrated optical waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a three dimensional see-through schematic showing theprinciples of the invention.

FIGS. 2 a and 2 b are partial schematic views of different embodimentsof a connecting structure as part of an optical waveguide structure toconnect the latter to a lateral, single mode waveguide on a chip.

FIGS. 3 a to 3 d are partial schematic views of different embodiments ofa connecting structure as part of a lateral, single mode waveguide on achip by which an optical waveguide structure is connected to thewaveguide on the chip

FIGS. 4 a and 4 b are partial schematic views of different embodimentsof connecting structure in the form of grating structures to connect anoptical waveguide structure to a lateral, single mode waveguide on achip.

FIG. 5 is a schematic view showing the trajectory of an opticalwaveguide structure (photonic wire bond) between light exit locations ofoptical connecting structures.

FIG. 6 is a schematic showing another trajectory of a photonic wire bondbetween light exit locations of connecting structures.

FIGS. 7 a, 7 b, 8 a, 8 b, and 9 are schematics showing differentembodiments for optical waveguide structuring methods that could be usedto implement the invention.

FIGS. 10 to 15 show an example of method steps that could be used tostructure optical waveguides to connect lateral single mode planarwaveguides according to the invention.

FIG. 16 is a schematic showing an embodiment of a device for producingphotonic wire bonds according to the invention.

FIG. 17 is a schematic showing another embodiment of a device forproducing photonic wire bonds according to the invention.

FIG. 18( a) is an electron microscope recording of an optical connectingwaveguide and FIG. 18( b) is a light-microscopic recording of severalphotonic wire bonds, realized on a joint chip is shown in FIG. 18( a).

DETAILED DESCRIPTION

In the drawings, like reference numbers are used to denote likecomponents in the different figures.

The waveguide structures upon which the invention is based are intendedfor the low-loss optical connection of lateral single-mode planarintegrated photonic systems. The functional principle of the inventionis explained with the example of the arrangement illustrated in FIG. 1.The optical arrangement (1) consists of several photonic systems thatare planar integrated on different substrates (2-4) (“chips”) and whichcontain lateral single-mode waveguides (50). The waveguides (50)positioned on different chips are to be interconnected and/or are to beconnected to output and input waveguides (5-6) (see also FIGS. 12-15),only input/output waveguide (6) being shown in FIG. 1. For this, theelements (2-6) to be connected are mounted on a component carrier (10)and are covered at least partially with a resist material (11). Each ofthe elements to be connected comprises optical connecting points (15-18)to which the connecting optical waveguide structures, also referred toherein as photonic wire bonds, (20-22) are attached. The preciseposition of the elements to be connected and connecting points isdetected with a measuring system. Based thereon, a favorable geometry isdetermined for the connecting waveguides which permits, for example, asingle-mode operation with the lowest possible propagation losses. Thisgeometry is converted to a digital dataset and the structuring of theconnecting optical waveguides structures (20, 21, 22) then occurs in thevolume of the resist material in accordance with this dataset and withthe aid of a direct-writing lithographic method. This method can bebased on polymerization reactions induced in the focused beam of ashort-pulse laser (30) through multi-photon processes. In a furtherprocessing step, the structures defined in this way can be exposedthrough developing the resists material and can be operated asfreestanding waveguides with the ambient air functioning as claddingmaterial. However, following the development they can also be embeddedin low-refractive cladding materials and in this way can be stabilizedmechanically or protected against physical and chemical environmentalinfluences.

There are numerous materials that can be used for the component carrier(10), depending on the application. In a simple case, it could, e.g.,just be a piece of metal that acts as a mechanical carrier and as a heatsink for the optical devices. The chips are then directly contacted bymetal wire bonds for electrical connections. Alternatively, a siliconsubstrate can be used as a carrier. This substrate can be equipped withelectrical circuitry (“intelligent CMOS substrate”), power distributionlines and other structures micro-electro-mechanical devices (MEMS). Whenused for sensor applications, the carrier may consist of a polymersubstrate that also contains microfluidic devices. For on-board opticalinterconnects, a polymer-based optoelectronic PCB can serve directly asa carrier. Photonic wire bonds are then used to connect on-chip devicesto board-level interconnect waveguides, hence avoiding high-precisionalignment in the board assembly process. Alternatively, dedicatedoptoelectronic carriers can be used as an intermediate platform thatcarry several optoelectronic devices and that are mounted to theoptoelectronic board. Photonic wire bonds can then be used both forconnecting the on-chip devices to the carrier and for opticallyconnecting the carrier to the board. In some cases it might bebeneficial to choose a carrier material for which the thermal expansioncoefficient is similar to that of the optical chips.

Photonic wire bonding is mainly a back-end-of-line-technology (BEOL),i.e., it is usually applied to chips that already have left the CMOSmanufacturing process. For many applications, it is hence not necessaryto use carrier materials that are compatible with CMOS processing.

The photonic wire bonds (20-22) produced according to this principle canhave a nearly optional three-dimensional shape which, in particular, canalso be adapted to the position and orientation of the planar integratedwaveguides to be connected. A highly precise positioning of integratedoptical components is thus no longer needed and the automatic productionof photonic wire bonds is made possible in high piece numbers. The indexcontrast can be freely adjusted over a large range when selecting asuitable cladding material. Extremely compact connection waveguides withsmall curve radii and high integration densities, in particular, can berealized in this way.

A measure for the optical losses, caused by the photonic wire bonds, isthe waveguide-dependent insertion loss which is measured between thebasic modes of the planar integrated lateral single-mode waveguides (50,51, 52, 53, 54) to be connected (see FIGS. 1-5). This loss shouldpreferably measure less than 6 dB, especially preferred less than 4 dBand particularly preferred less than 2 dB. These values shouldadvantageously be maintained over an optical band width of more than 30nm, especially more than 100 nm and particularly preferred more than 300nm, wherein high integration densities and an efficient production withhigh piece numbers should also be possible. The minimum distance betweentwo photonic wire bonds which can be realized on the same substrate isadvantageously less than 200 μm, especially less than 30 μm andparticularly preferred less than 10 μm. Methods disclosed in the priorart do not meet these requirements. In the following, various featuresof the invention are explained further which allow overcoming therestrictions according to the prior art.

The three-dimensional form of the photonic wire bonds (20, 21, 22) isadapted to the spatial position of the planar waveguides to be connected(50, 51, 52, 53, 54). In contrast to planar integrated waveguidestructures, photonic wire bonds are distinguished by a course or paththat differs from the integration plane of the planar waveguides to beconnected (see FIG. 1). As a result, a coupling in and coupling out viathe substrate surface is possible, as well as to overcome the heightdifferences between the surfaces of different substrates.

Compact optical waveguide structures (20, 21, 22) furthermore requiresmall curvature radii. To keep radiation losses to a minimum, asufficiently high index contrast Δn=n₁−n₂ is required between therefractive index n₁ in the core of the waveguide and the refractiveindex n₂ in the sheath for the waveguide. For wavelength ranges between1300 nm and 1600 nm, the refractive index of the sheath material ispreferably between 1 and 2.8, especially between 1.2 and 1.8 andpreferably between 1.3 and 1.6. The minimum requirement for the indexcontrast should be that the insertion losses for an 180° turn with 4 mmcurve radius measures less than 1 dB. For this, the refractive indexcontrast Δn between core and sheath region of the photonic wire bondmust measure at least 0.01. However, the index contrast shouldpreferably be higher than 0.05 and especially higher than 0.15. Withfreestanding structures, the Δn can even amount to more than 0.3.However, with high index contrasts the requirements to be met by thestructuring method can increase strongly (resolution, achievable surfaceroughness). In general, the possible minimum curvature radii depend onthe index contrast and the cross-sectional geometry of the photonic wirebond. The optical waveguide structures (20, 21, 22) are designed suchthat local curvature radii of preferably less than 10 mm or 5 mm,especially less than 500 μm and especially preferred less than 50 μm canbe realized. In individual cases, the curvature radii can be even lessthan 10 μm.

A low-loss optical connection between the lateral single-mode waveguides(5, 6, 50, 51, 52, 53, 54) connected to the photonic wire bonds can beachieved, for example, with waveguide structures (20, 21, 22) which arealso single mode, wherein the lateral dimensions of the waveguide coremust be selected to be sufficiently small. The maximum core diameter 2 a(see FIG. 5) in this case depends on the refractive indexes in the coreand sheath region. With wavelengths in the range of 1550 nm and straightor nearly straight waveguide sections having circular or nearly circularcross sectional geometries, the lateral core diameter preferablymeasures less than 10 μm or 7 μm, especially less than 5 μm and inparticular less than 3 μm. With freestanding structures having a highindex contrast, the core diameter can even be less than 1.6 μm. Withwavelengths in the range of 1300 nm, these dimensions are preferablyreduced to less than 8 μm or 6 μm, especially preferred less than 4 μmand in particular less than 2.5 μm, wherein for freestanding structuresit can be less than 1.3 μm.

A low-loss optical connection, however, can also be obtained withlateral multi-mode waveguide structures (20, 21, 22). In that case, itmust be ensured with the aid of connecting structures (23, 24, 55, 56,57, 58, 59) and/or with a suitable waveguide shape that a low-losstransformation occurs between the basic modes of the lateral single-modewaveguides (5, 6, 50, 51, 52, 53, 54) connected to the photonic wirebond, wherein a numerical optimization of the waveguide structure isgenerally required for this. For example, multimode interference (MMI)effects in the photonic wire bond (20, 21, 22) can be utilized purposelyfor this. Alternatively, a suitable design of the connecting structures(23, 24, 55, 56, 57, 58, 59) can also be used to ensure that the basicmodes of the single-mode waveguides (5, 6, 50, 51, 52, 53, 54) will onlyexcite the basic mode of an intrinsically multimode photonic wire bond.

With curved waveguide sections, the wave guidance can also take the formof a so-called whispering gallery mode which propagates along a convexinterface between a core material and the sheath material. Thesingle-mode wave guidance is possible in that case, even if the lateraldimensions are clearly above the aforementioned values (see FIG. 5)and/or if the waveguide is defined only by the curved outside contour;see FIG. 6. Typical lengths for the waveguide structures (20-22) arebetween 10 μm and 30 mm, especially between 30 μm and 3 mm and inparticular between 50 μm and 1 mm.

The connecting structures (23, 24, 55, 56, 57, 58, 59) ensure a low-losstransition between the photonic wire bond (20, 21, 22) and the connectedsingle-mode waveguides (5, 6, 50, 51, 52, 53, 54). In particular this isimportant if the planar integrated photonic subsystems to be connectedare based on semi-conductor optical waveguides with high refractiveindex contrast and correspondingly small mode field diameters of a fewhundred nanometers while the connecting waveguides are characterized bysmall to average index contrasts and correspondingly higher mode fielddiameters of up to several micrometers. The design of the transitionbetween a planar integrated waveguide (50, 51, 52, 53, 54) and aphotonic wire bond (20-22) strongly depends on the respectivecross-sectional geometries. With sufficiently small differences for theindex contrasts, a coupling can be realized simply with a tapering orexpanding taper section (23, 24) in the connecting waveguide (see FIGS.2( a) and 2(b). With high index contrasts, special structures foradapting the mode field must also be provided on the side of theintegrated waveguide (see FIGS. 3( a), 3(b), 3(c), 3(d) and 4(a), 4(b).

FIG. 2( a) outlines the example of a connecting waveguide (20) with anexpanding taper section (23), which results in a mode field adaptationto a standard single-mode fiber (50) with small index contrast. Modefield adaptations to integrated waveguides (50) with high index contrastcan be achieved by a design where the taper section narrows (24) asshown in FIG. 2( b). In addition, planar integrated waveguides (50, 51,52, 53, 54) can also be provided with connecting structures whichinfluence the mode field diameter and/or the propagation direction forthe light at the connecting location. FIG. 3( a), for example, shows anexample of a planar integrated waveguide (50) for which the mode fieldis expanded by a tapered structure (55) in one direction and is thusadapted to an adjoining connecting waveguide (20). An expansion of themode field can also be achieved with a narrowing tapered waveguidestructure (56) (a so-called “inverse taper”) as shown in FIGS. 3( b) and3(c)). Alternatively, as shown in FIG. 3( d) a periodic structure (59)can also be used for coupling the guided modes of the integratedwaveguide (50) into the connecting waveguide (20). The form, periodicityand length of the connecting structure are selected so as to ensure themost efficient mode conversion.

The inverse taper shown schematically in FIGS. 3( b) and 3(c) representsa preferred connecting structure for the connection between a photonicwire bond (20) and a planar integrated silicon-on-insulator (SOI) stripwaveguide (50). The refractive index in the core of the siliconwaveguide in this case is approximately 3.48 for a wavelength of 1550nm. As a rule, the waveguide has a nearly rectangular cross section witha preferred height between 200 nm and 500 nm, especially between 200 nmand 350 nm. The width w₁ of the integrated waveguide (50) is preferablybetween 200 nm and 800 nm, especially preferred between 300 nm and 500nm. In the connecting structure (56) embodied as inverse taper to thephotonic wire bond (20), this width is reduced to values w₂ ofpreferably less than 120 nm, especially less than 80 nm and inparticular less than 60 nm. The length L₁ for the inverse taper ispreferably between 10 μm and 400 μm, especially preferred between 15 μmand 200 μm and in particular between 20 μm and 120 μm. The photonic wirebond (20) has a width w₃ which preferably ranges from 0.5 μm to 10 μm,especially preferred from 1 μm to 5 μm and in particular from 1.5 μm to4 μm. The width w₄ for the taper (24) is preferably smaller than orequal to the width w₃ and is larger than or equal to w₁. Numericalvalues for w₄ therefore are preferably between 200 nm and 10 μm,especially preferred between 300 nm and 4 μm and in particular between500 nm and 2 μm. The length L₂ of the taper (24) is preferably between10 μm and 400 μm, especially between 15 μm and 200 μm and in particularbetween 20 μm and 120 μm.

For the periodic connecting structure shown in FIG. 3( d), the periodlength A with an operating wavelength of approximately 1550 nmpreferably amounts to more than 680 nm, especially more than 750 nm andespecially preferred more than 950 nm. For a wavelength of 1300 nm,these values are reduced to 570 nm, 630 nm and 800 nm.

A connecting waveguide (20) can furthermore be connected to a planarintegrated waveguide (50) via a grating structure (57) as shown in FIG.4( a). The grating causes the light to be radiated at a large angle,relative to the substrate plane, so that the connecting waveguide (20)can be led away in a space saving manner directly toward the top,similarly to a bond wire used in the electrical design and connectiontechnology. With a grating (58) that is structured in two directions,the two linear polarization states of the optical connecting waveguidecan be divided into identical polarization states for two differentplanar waveguides (53) and (54) as shown in FIG. 4( b). Dual structuresof this type can be of critical importance for future opticalcommunication systems, in which both polarizations are used for the datatransfer.

For an efficient connection of the photonic wire bond (20) via gratingstructures (57, 58), the grating must have a certain minimum number ofelements that repeat periodically per se. The number of periodicallyrepeating grating elements preferably is between 4 and 40, especiallybetween 6 and 30 and particularly preferred between 8 and 25. Thegrating elements can repeat in one (FIG. 4( a) or two directions(Figures (b)). The total length and/or width of the grating preferablymeasures between 1 μm and 20 μm, especially between 2 μm and 16 μm andespecially preferred between 3 μm and 10 μm. The radiation angle ispreferably between 0° and 45°, especially between 5° and 30° andparticularly preferred between 5° and 15° (respectively measuredrelative to the direction of the normal to the substrate plane).

As described in more detail below in connection with FIGS. 12 and 16, ameasuring system (70) is used to detect the spatial position andorientation of the optical elements (2-6) mounted on the componentcarrier (10) and/or of the associated connecting locations (15-18). Thismeasuring system can be based, for example, on an interferometry methodfor the highly precise axial position measuring or a camera-based methodfor the lateral position detection and the identification of theconnecting points. In addition, laser scanning methods can be used whichscan the interface between the elements to be connected and thesurrounding resist material and which, in the process, detect theposition of the surface as well as the precise spatial position of theindividual connecting points. Confocal microscopic techniques, forexample, also offer themselves as further options for providing ahigh-resolution, three-dimensional dataset for the optical interfaces ofthe arrangement from which the position of the elements to be connectedand the connecting locations can be extracted. In place of a directdetection of differently designed connecting locations or points,alignment marks (40) with defined relative position to the connectinglocation can also be provided on the optical elements to be connected,wherein the position of these markers can be detected by the measuringsystem and can be used for the spatial positioning of the connectionwaveguides to be generated; see also FIGS. 3 and 4. The measuring systemused for detecting the position of the optical elements to be connectedmust have sufficiently high accuracy and resolution to ensure areproducible positioning of the connecting waveguide (20), relative tothe connecting structures (55-58). On the other hand, a sufficientlylarge measuring range is required so that both end points of thewaveguide trajectory can be measured within one and the same coordinatesystem. The absolute precision of the measuring system should preferablybe better than 1 μm, especially preferred 500 nm and in particular 50nm. The spatial resolution should advantageously be better than 1 μm,especially better than 500 nm and particularly preferred better than 200nm. The measuring range that can be detected with one and the samecoordinate system in this case preferably extends over more than 100 μm,preferably more than 500 μm, and especially preferred more than 2 mm.

Referring to FIGS. 5 and 6, the shape of a connecting waveguide (20) isadapted to the spatial position of the regions (16, 17) to be connectedand the propagation directions for the light at these locations. Thelocal propagation direction for the light at the connecting locations(15-18) is described by the directional vectors (63, 64) and depends onthe design of the connecting structures (55-58), wherein the light exitlocations are marked by the points (61), (62). The geometry of thephotonic wire bond (20) is then primarily determined by the course ofthe waveguide trajectory (60) between the points (61) and (62). Thetrajectory (60) is preferably selected such that it assumes thedirection predetermined by the direction vectors (63, 64) at the endpoints while the integral optical losses, occurring along the photonicwire bond (20), are minimal.

On the one hand, these propagation losses are caused by the intrinsicloss mechanisms of the waveguide (e.g. losses through materialabsorption, scattering losses on rough surfaces), wherein this shareincreases with the length of the waveguide trajectory (60). Added tothis are the radiation losses which are caused by the curvature of thetrajectory and, accordingly, depend on the refractive index contrast ofthe connecting waveguide.

The intrinsic losses are characterized by a loss coefficient α whichdepends on the material and waveguide cross section. The integral amountoccurring along the waveguide trajectory can be minimized by selectingthe shortest possible waveguide trajectory. In contrast, the radiationlosses of the waveguide strongly increase with the increase in thecurvature K. To minimize the integral radiation losses, small curvatureradii should be avoided which, as a rule, leads to long waveguidetrajectories, so that conflicting goals exist for minimizing theintrinsic losses and the radiation losses. This conflict can be solvedoptimally with a specific waveguide trajectory. The course of thistrajectory (60) can be determined through minimizing a loss function I,for example having the following form:

I({right arrow over (r)}(s))=∫₀ ^(L)(α+f(K(s))ds   (1)

With P_(out)=P^(in)·e⁻¹, where P_(out) is the optical output-power,whereas P_(in) is the optical input-power. In this case, {right arrowover (r)}(s) represents the course of the curve in space asparameterized according to the curve length s; K(s) represents thecurvature of the trajectory; and f represents a monotonously growingfunction describing the dependence of the radiation losses on the localcurvature. The minimizing of the function based on the equation (1),with the marginal conditions predetermined by the end points (61, 62)and the direction vectors (63, 64), as a rule is made numerical andsupplies the optimal course for the waveguide trajectory (60), in viewof the integral optical losses.

Additional criteria can be used when designing the trajectory, e.g.avoiding collisions with other structures, the mechanical stability ofthe freestanding waveguide structure or the writing time that isrequired for producing the photonic wire bond (20) with a specificdirect-writing lithographic method. These criteria are reflected in theadditional marginal conditions used for minimizing the loss functionaccording to equation (1). Tapered structures in the region oftransition to the connecting points must be considered when determiningthe ideal trajectory.

In addition to the trajectory (60) of the connecting waveguide, thecourse of the cross-sectional geometry along this trajectory also playsan important role because it determines the shape of the core-sheathinterface for the optical waveguide (65). The cross sectional geometry,for example, can assume round, elliptical or polygonal forms. For manyapplications it is advantageous if the photonic wire bond (20) has twolinear main polarization states with the largest possible optical bandwidth, wherein non-rotation symmetrical, for example elliptical orrectangular, cross sectional profiles can be used for this. In photonicsystems using polarization multiplexing methods, the course of thecross-sectional geometry is preferably selected such that the photonicwire bond (20) has two linear polarized main polarization states whichare transitioned via corresponding connecting structures (23, 24, 55,56, 57, 58, 59) to the two main linear polarization states of theconnected, integrated optical system.

In addition to standard waveguide structures, for which wave guidance isachieved with a refractive index contrast between core and sheathregion, waveguide structures can also be used that guide the light alonga convex dielectric interface in the form of so-called whisperinggallery modes. A structure of this type is sketched, for example, inFIG. 6, wherein the trajectory (60) that is relevant for the lightguidance moves along the convex surface (65), at a distance determinedby the local curvature of the surface.

A high-resolution, direct-writing lithographic method is used for thethree-dimensional structuring of the photonic wire bonds (20, 21, 22),which can be based on the interaction between electromagnetic radiationand the resist material (11, 12) that results in a thermally orphoto-chemically induced change in the material.

With reference to FIGS. 1 and 11-15, resist materials (11, 12) can bedesigned such that as a result of the structuring with thedirect-writing lithographic method, the refractive index is increasedand an optical waveguide is thus generated which is embedded intonon-exposed resist material with a lower refractive index. Following theexposure, the resist structure is normally fixated by removing, forexample, the photo-sensitive component from the material. In this way,embedded waveguide structures are obtained which are distinguished byeasy production and high mechanical stability. However, it is difficultto achieve index contrasts of more than 0.01 with the material systemsavailable today.

According to one embodiment, the non-exposed regions of the resistmaterials (11, 12) are removed during a development step following theexposure. The remaining structures can either be used directly asfreestanding waveguides with high index contrast against the ambientair, or they can be embedded in a further step into a cladding material(25) with low, essentially freely selectable refractive index.

Light in the ultraviolet, visible, infrared wavelength or X-ray range,can be used for the lithographic definition of the waveguide structures(20, 21, 22). A specific intensity distribution of the electromagneticradiation in the volume for the resist material must be generated toproduce defined structures. Three-dimensional structures with extremelyprecisely defined geometries can be achieved with multi-photon processesfor which the degree of material modification depends non-linearly onthe local intensity of the light, wherein these methods include, forexample, the two-photon polymerization. For the most part, pulsedradiation sources are used for which the maximum optical output ishigher by several orders of magnitude than the average output in orderto reach the optical intensities necessary for the multi-photonprocesses.

Three-dimensional structures can be generated, for example with a laserbeam (30), for which the focus (31) (see FIG. 1) has the requiredintensity for changing the material. The resist material can thus bemodified at precisely defined locations in the volume. As a result ofthe controlled movement of the focal point within the resist volume,many such point-shaped structural elements (“voxels”) can be linkedtogether and complex three-dimensional structures can be built. In theprocess, different writing strategies can be used which depend on thekinematics used and the properties of the resist material. With resistmaterials which are viscous or liquid during the writing operation, amovement (“swimming away”) of already exposed structural regions withinthe resist volume must be prevented. Writing strategies therefore offerthemselves for which the connecting optical waveguide is configuredstarting from the connecting locations (16, 17), without isolated oronly weakly anchored partial regions being generated during the writing.

Different exemplary writing strategies are illustrated in FIGS. 7( a),7(b) and 8(a), 8(b). A very simple writing strategy is based, forexample, on the division of the three-dimensional waveguide structureinto separate, mostly parallel layers (80) which are combined in layersalong a specific direction as shown in FIG. 7( a). Each of these layersin turn consists of straight, mostly parallel lines, each of which isconfigured with individual, point-type structural elements (voxels).Alternatively, the three-dimensional waveguide structure can also beconfigured with individual disks (81) which are oriented perpendicularto the center line (60) of the waveguide as shown in FIG. 7( b). Thedisks can be split up, for example, into straight or into concentricelliptical lines. Alternative thereto, they can also be written in theform of continuous spirals. With the methods schematically shown inFIGS. 7( a), 7 (b) and 8(a), 8(b), the waveguide is configured startingwith the connecting locations (16) and (17) and is thus at all points intime fixedly connected to the substrates (2, 3, 4) and/or the input andoutput waveguides (5, 6). Writing strategies of this type can also beused with viscous or liquid resist materials. However, the strongcurvatures of the writing trajectories are a disadvantage of thesemethods. The curvature radii are primarily determined by the radius ofthe waveguide structure and can assume values of less than 1 μm. Sincethe maximum lateral acceleration when moving along a trajectory islimited in the upward direction by the kinematics of the lithographysystem, strongly bent curves can be written only at very low speed. Inview of the writing time, writing strategies are therefore advantageousfor which the waveguide structures are composed of individual, ifpossible only slightly curved trajectories, wherein this can be achievedif the writing trajectories extend in a longitudinal direction of thewaveguide as shown in FIGS. 8( a) and 8(b). The three-dimensionalwaveguide structure can thus be configured with individual and uniformlyspaced-apart lines, for example extending parallel in a longitudinaldirection as shown in FIG. 8( a). These lines start and end either atthe connecting regions (16) and (17) or at the core/sheath interface(65) of the waveguide. Alternatively, the waveguide can also beconfigured with individual lines for which the lateral distances d alongthe waveguide are adapted to a differing cross section as shown in FIG.8( b). With this method, all writing trajectories start and end at theconnecting regions (16) and (17). As a result, a very smooth waveguidesurface (65) can be achieved which minimizes the propagation lossescaused by optical scattering. With the method sketched in FIG. 8( b),the exposure dose along the writing trajectories can be dynamicallyadapted to the local spacing for the exposure trajectories to ensure acomplete exposure of the material for long distances, without risking anexcessive exposure for short distances. In addition to the methodsoutlined in FIGS. 7( a), 7(b) and 8(a), 8(b), other writing strategiescan also be used, for example based on a combination of the abovedescribed methods. With waveguide cross sections that vary strongly inthe longitudinal direction, for example, the internal regions can beconfigured with parallel lines and non-continuous line sections as shownin FIG. 8( a) while regions, which are close to the surface (65), areconfigured with continuous lines arranged at varying distances. Writingstrategies can also be realized for which only an outer sheath of thewaveguide to be produced is written on. The non-exposed materialsurrounded by the sheath can also be polymerized and thus solidified ina subsequent step using flood light. This method has the decisiveadvantage of resulting in a time saving since the writing of the linesclose to the axis can be omitted. The challenge with this writingstrategy is in the production of a dense sheath surface.

Alternative to configuring complex three-dimensional structures withindividual, identical structural elements (voxels), thethree-dimensional structuring can also be realized with a laser beamwhere the form of the field distribution in the focal region is adjusteddynamically with the aid of an adaptive optical unit. It is thuspossible to continuously adapt the shape of the structural elementgenerated with the laser beam during the writing operation. With thismethod it is possible, for example, to produce a complete photonic wirebond during a single operational step as follows: The region ofinteraction is moved along the waveguide trajectory (60) through theresist material (11, 12); the cross-sectional geometry of the waveguidecan be adjusted at any point through the dynamically adapted voxel form.The resolution for this method is no longer determined solely by thevoxel size, but depends in a complex manner on the adaptive opticalsystem that is used. To permit a reliable connection of the photonicwire bond to single-mode waveguides, however, such a method must alsoallow producing structures with lateral dimensions of preferably lessthan 4 μm, especially less than 2 μm and especially preferred of lessthan 1 μm.

Alternatively to the above-described “scanned” structuring operations,the interferometry (or holographic lithography) techniques can also beused, which generate three-dimensional interference patterns with theaid of targeted super-imposition of coherent light waves. These patternsdefine the form of the waveguide (20) in some sections or completely inthe volume of the resist material (11). A method of this type isschematically illustrated in FIG. 9. An amplitude-phase modulation unit(74) converts the three-dimensional structure information of thewaveguide to the amplitude and phase course of a coherent light field.FIG. 9 shows a coherent input field (35), for example a planar lightwave and an illumination field (36) which comprises thethree-dimensional structure information for the waveguide. In FIG. 9,the illumination field (36) comprises a delimited spatial angle.However, with suitable optical arrangements, a much larger spatial angleregion can also be used for the lithography, for example by introducingan additional light path which permits a coherent illumination of thesample from above and below. For methods of this type, techniques fromthe field of high-resolution microscopy such as the so-called 4mmicroscopy can be used.

The direct writing lithographic method must have sufficient accuracy andspatial resolution to allow photonic wire bonds to be connected tolateral single-mode waveguide structures. This accuracy with whichindividual structural elements can be placed in space is preferablybetter than 1 μm, especially better than 200 nm and particularlypreferred better than 50 nm. Independent of the lithographic method, theresolution is preferably higher than 2 μm, especially higher than 1 μmand particularly preferred higher than 200 nm. When using scanning 3Dlithographic methods operating with a beam, the generated structuralelements (voxel) frequently are not point-shaped, but have a nearlyellipsoidal shape. In that case, the spatial anisotropic resolution ofthe lithographic method can already be taken into consideration duringthe design of the waveguide structure. The aforementioned accuracies andresolutions are maintained over a range which preferably extends overmore than 100 μm, especially more than 500 μm and particularly preferredmore than 2 mm.

Individual aspects of the method for the optical connection of planarintegrated lateral, single-mode waveguides are discussed above. Themethod steps are shown with examples in FIGS. 10-15 and are disclosedonce more in the following, wherein the representation below must beviewed as exemplary.

The method for producing photonic wire bonds comprises the followingsteps:

-   -   1. In a first step, the photonic chips (2-4) to be connected and        possibly existing input and output waveguides (5, 6) are mounted        on a joint component carrier (10) as shown in FIG. 10. No        special precision requirements must be met since the precise        position of the elements to be connected is detected during the        course of realizing the method and the geometry of the        connecting waveguides is adapted thereto. The component carrier        itself can also contain waveguides (28) which form a transition        between the input and output waveguides and the photonic        components to be connected.    -   2. In a second step, the regions of the elements to be connected        are covered with a resist material (11, 12) as shown in FIG. 11.        For the subsequent contacting of photonic components with        standard metal-wire bonds, non-covered regions (19) can also be        provided. The second step also involves possible pre-treatments        of the resist materials, for example a pre-curing under the        effect of heat.    -   3. In a third step, the spatial position and orientation of the        optical waveguides (50, 51, 52, 53, 54) to be connected and/or        the input and output waveguides (5, 6) are detected with a        measuring system (70), relative to a machine coordinate system        (71) as shown in FIG. 12.    -   4. In a fourth step, a favorable geometry is determined for each        of the photonic wire bonds (20, 21, 22) on the basis of the data        determined during the third step. The trajectory of the        waveguide is selected such that the photonic wire bond connects        the respective connecting locations and that the waveguide is        oriented at the starting and end points in the direction        predetermined by the connecting structure and does not overlap        with other structures. In addition to the trajectory of the        waveguide, a favorable course of the waveguide cross section        along the trajectory is determined. The precise form of the        trajectory as well as the course of the waveguide cross section        can be determined with the aid of various optimization criteria        which include, for example, the optical losses of the connecting        waveguide or the writing time required for producing the        waveguide with a direct-writing lithographic method. According        to one writing strategy, the three-dimensional form of the        waveguide is converted to a machine-readable dataset.    -   5. In a fifth step, the connecting waveguides (20, 21, 22) are        defined with the aid of a direct-writing lithography device (72)        as shown in FIG. 13. The achievable resolution in this case is        approximately 2 μm or less. The lithography system uses the        machine coordinate system (71), used for the position measuring        in step 3, and/or a sample coordinate system derived therefrom.        When combining, an optical measuring method with an optical        lithography method, parts of the optical arrangement can be used        for the position determination as well as for structuring the        waveguide, thereby making it possible to minimize the influence        of errors in the optical system.

With the example shown in FIG. 13, the optical structuring of theconnecting waveguides in the volume of the resist materials (11, 12)occurs with a focused laser beam that is radiated onto the surface ofthe resist materials and which modifies the material at its focal point.In practical operations, it is frequently difficult to realize resistlayers (11, 12) with sufficient thicknesses which simultaneously alsohave a surface with optical quality (flatness). This problem can becircumvented with the arrangement shown in FIG. 14. The opticalstructuring occurs with the aid of a transparent element (75) which hasa defined interface with optical quality to the material (12). Theoptical element can be placed onto the component carrier 10, asillustrated in FIG. 14, in the form of a “cover glass” or it can form acomponent of the component carrier 10 (“transparent writing window”).Alternatively, the surface of the resist layer can be determined in step3 and can be considered during the lithographic conversion of thewaveguide structure.

-   -   6. In a sixth step, the structured resist materials (11, 12) are        subjected to a post-treatment, wherein this step comprises, for        example, one or several of the following processes:        -   fixation of the exposed resist structure, e.g. with the aid            of a thermal treatment;        -   removal of the non-exposed regions of the resist material            during a development step.    -   7. In an optional seventh step, the produced waveguide        structures are post-treated, which can improve the optical light        guidance or the physical or chemical stability of the waveguide        structures. This step involves, for example, one or several of        the following processes:        -   depositing a coating (14) onto the surface of produced            waveguide structures, for example by precipitating out a            coating material from a gas phase (chemical vapor deposition            or CVD; atomic layer deposition ALD) or from a liquid phase;    -   embedding of freestanding structures in a cladding material with        low refractive index.

The example in FIG. 15 illustrates a structure for which the non-exposedregions of the resist material (11, 12) were removed during the step 6.The freestanding connecting waveguides are initially provided with aprotective layer (14) and are then embedded into a low-refractivecladding material (25).

Connecting photonic wire bonds (20, 21, 22) to planar integrated,lateral single-mode waveguides requires a precise relative positioningbetween the substrates (2, 3, 4) and/or the associated opticalconnecting locations (16, 17, 18) and the lithographically producedwaveguide structures (20, 21, 22). The accuracy over the total operatingrange for the relative positioning is preferably better than 1 μm,preferably better than 200 nm and especially preferred better than 50nm.

A close linking of the measuring system and the lithographic system isnecessary to meet these accuracy requirements. FIG. 16 schematicallyillustrates an example of a corresponding device for producing photonicwire bonds which comprises a holding device (76) for the opticalarrangement (1) mounted on the component carrier (10), a measuringsystem (70) for determining the spatial position and orientation of theelements to be connected and their connecting locations, relative to amachine coordinate system (71), a direct-writing lithography system (72)which allows the three-dimensional structuring of connecting waveguideshaving a resolution better than 2 μm, and a data processing unit (73).The holding device (76) may comprise kinematics or mechanisms whichpermit a precise spatial positioning of the optical arrangement (1),relative to the measuring system (70) or to the lithography system (72).Alternatively, the measuring system (70) and the lithography system (72)can also be provided with internal positioning devices (e.g. scanningmirrors) which can be used to move the measuring or writing beamrelative to the optical arrangement.

A data processing unit (73) controls the sequence of the above-describedproduction method steps. The data processing unit (73) furthermorecomputes a favorable three-dimensional geometry for the connectingwaveguides, based on the measuring data provided by the measuring system(70), and converts this geometry with the aid of a suitable writingstrategy to a dataset which is then used by the lithography system asbasis for structuring the waveguide.

The realization according to the invention of the herein describedphotonic wire bonds can be based, for example, on the structuring of aphotoresist from the Su8 family (company Microchem Corp.) withtwo-photon polymerization. A mode-coupled laser may be used in this caseas a light source for the lithography, wherein this laser emits opticalpulses with a pulse width of 120 fs and a repetition rate ofapproximately 100 MHz at a wavelength of 780 nm. The pulsed laser beamis focused with an objective having a high-numerical aperture (100×,NA=1.4) onto the resist material Su8. Owing to the high peak intensityof the pulsed laser beam, a two-photon polymerization takes place in anellipsoidal region at the focal point. By moving the sample (opticalarrangement (1)) in a lateral direction, relative to the focal point,individual ellipsoidal points can be combined to form complexthree-dimensional structures. The positioning of the writing beamrelative to the sample is achieved, for example, through piezopositioning tables which can have traversing ranges of several hundredmicrometers and repeatability of less than 10 nm. The lateral resolutionof the lithography system perpendicular to the writing beam is providedby the beam diameter in the focus and ranges from 150 nm to 500 nm,depending on the selected dose. In the axial direction, the resolutionis between 600 nm and 1500 nm. The laser output must be adapted to thewriting speed in that case. For a linear relative movement withapproximately 200 μm/s for the sample perpendicular to the axis of thewriting beam, writing capacities ranging from 10 mW and 20 mW aretypically used.

The waveguides to be connected are integrated silicon-on-insulator (SOI)waveguides having a rectangular cross section of 220 nm in height andapproximately 400 nm in width. Inverse tapers are used for theconnecting structures, wherein the width of these tapers is reduced toapproximately 60 nm over a distance of, for example, 30 μm. A confocalmeasuring method is used to determine the position of the connectingstructures. The same optics and mechanics which are also used for theillumination are essentially used for the measuring operation. Possibleimage errors in the optical equipment and/or position errors in themechanical equipment will therefore self-compensate, at least in part.Using a microscope objective with the aforementioned numerical aperture,lateral regions of typically up to 100 μm can be detected in the broadfield without having to move the sample. Typical resolution values areapproximately 150 nm in the lateral and <100 nm in the axial direction.The repeatability of the position determination for broad structures canbe better than 10 nm.

The waveguide trajectory is determined by numerically minimizing theloss function according to the equation (1). The connecting waveguidesare structured in the longitudinal direction, meaning by producingindividual parallel lines that extend in the longitudinal direction. Theconnecting waveguides have an elliptical cross section with a largesemi-axis of approximately 2 μm and a small semi-axis of approximately 3μm. Following the exposure, the resist material is developed. Thewaveguide structures that are exposed in the process are then embeddedin a low-refractive cladding material (e.g. a fluorinated polymer whichis commercially available under the brand name CYTOP). At 1550 nm, Su8has a refractive index of approximately 1.57 while the refractive indexof the cladding material can be at 1.4. With the aforementionedelliptical cross section, an efficient light guidance is possible onlyfor two basic modes that are polarized orthogonal to each other, whereinthe ellipticity causes an uncoupling of these modes. Higher waveguidemodes are guided only weakly and therefore do not play a role worthmentioning for the light propagation. The waveguide is thereforeeffectively single-mode and permits an efficient connection to planarintegrated, lateral single-mode nano waveguides.

FIG. 17 illustrates an example of a device for producing photonic wirebonds. The positioning unit (76) here functions to spatially positionthe optical arrangement (1), consisting of component carrier (10) andthereon mounted substrates (2-4), within a machine coordinate system(71) that is jointly used by the measuring and the lithography system.To avoid distortions during the detection, measuring and lithographicstructuring of the arrangement (1), a telecentric beam path (108) on theobject side of an objective lens 101 is used for which an aperture (107)is located in the focal plane of the objective lens (101), on theimage-side. A camera (95) is used for detecting the optical arrangement(1) in the broad field. With the aid of a suitable calibration,positions in the camera image can be made to relate to coordinates inthe machine coordinate system (71). A highly precise, completelythree-dimensional detection of the position of the optical waveguide(50, 51, 52, 53, 54) to be connected and/or the input and outputwaveguides (5, 6), however, is possible only with the aid of a confocalmeasuring method. For this, the light from a light source (93) isfocused with a circular aperture (104) and is subsequently guided withthe aid of one or several scanning mirrors (106), a scanning lens (103)and a tube lens (102) to the objective (101) which focuses the light.The light scattered back at the focal point is focused into a circularaperture (105) by a detection beam path, arranged confocal to theillumination beam path, and is detected by a detector (94). With the aidof a lateral scanning of the illumination and detection beam path, thetotal focal plane on the objective side of the objective (101) can thusbe scanned. By displacing the optical arrangement (1) in the zdirection, a three-dimensional dataset can thus be detected whichpermits a precise spatial position detection of the optical elements tobe connected. The same optical arrangement is also used for thelithographic structuring of the single-mode connection waveguide (20-22)with two-photon polymerization. For this, a pulsed laser beam (111) iscoupled via a beam divider (110) and with the aid of the circularaperture (104) into the illumination beam path which is then focused bythe objective (101) onto the volume of the resist material (11, 12). Astructuring of complex, three-dimensional geometries is achieved, forexample, in that the lithography beam is moved by the scanning mirror(106) in the lateral direction while the photonic system (1) isdisplaced by the positioning unit in the axial direction. The pulsedlaser beam is generated with a light source (91) and isamplitude-modulated with a modulation unit (92). The completearrangement is controlled with a data processing unit (73) which is notshown in FIG. 17) to provide a better overview.

Two-photon polymerization allows for arbitrary three-dimensionalwaveguide geometries. By specific 3D waveguide routing algorithms,complex cross-routing of the signals is possible. In particular,photonic wire bonding enables low-loss on-chip waveguide crossings thatare hard to realize in conventional planar waveguide technology.

Limitations in waveguide geometry might be imposed by the mechanicalstability of freestanding photonic wire bonds, but structuresinvestigated so far turned out to be very sturdy with waveguides of 2 μMdiameter spanning distances of more than 100 μm.

Photonic wire bonds have already been produced and experimentally testedwith excellent results within the framework of the present disclosure,with the following results being obtained:

Within the framework of the initial work, a first functional photonicwire bond was realized. A lithography system by the company NanoscribeGmbH of Germany was used for the structuring. An electron microscoperecording is shown in FIG. 18( a). The references (50) and (51)designate the input and output waveguides; the reference (20) stands forthe optical connecting waveguide; the reference (57) shows the gratingconnector by means of which the optical connection is made. FIG. 18( b)shows a light-microscopic recording of several photonic wire bonds,realized on a joint chip, in the form of freestanding structures whichinterconnect single-mode Nano photonic silicon-on-insulator waveguides.The connecting waveguides were produced with a direct-writinglithographic system of the company Nanoscribe and a commerciallyobtainable photoresist of the Su8 family (company Microchem Corp. ofNewton, Mass.).

An improved photonic wire bond structure with excellent transmissionproperties has been realized in accordance with the present disclosure,having an insertion loss of approximately 3 dB over a wavelength rangeof approximately 300 nm have been demonstrated.

Although two-photon polymerization is a relatively young technology, noobstacles are seen to scale it up to mass-production environments.Writing speed can be significantly increased by using scanner-basedlithography systems, and fully automated metrology techniques allow forhighly precise alignment of photonic wire bonds with respect to on-chipwaveguides. Photonic wire bonds allow use of materials that are wellestablished in micro fabrication (e.g. SU8), so that no fundamentalproblems are expected in terms of compatibility and long-term stability.

The following articles authored by the inventors of this application andothers are incorporated herein by reference in their entireties: [1]Lindenmann, et al.: ‘Photonic Waveguide Bonds—A Novel Concept forChip-to-Chip Interconnects,’ Proc. Optical Fiber CommunicationConference (OFC'11), Los Angeles (Calif.), Paper PDPC 1, Mar. 6-10,2011; [2] Lindenmann, N et al.,; “Photonic Wire Bonding for Single-ModeChip-to-Chip Interconnects,’ 8th International Conference on Group IVPhotonics, London, England, Sep. 14-16, 2011; paper FD2; and [3]Lindenmann, N. et al.; “Photonic wire bonds for terabit/s chip-to-chipinterconnects;” published at ArXiv.org on Nov. 2, 2011.

The invention has been described in detail with respect to variousembodiments, and it will now be apparent from the foregoing to thoseskilled in the art, that changes and modifications may be made withoutdeparting from the invention in its broader aspects, and the invention,therefore, as defined in the appended claims, is intended to cover allsuch changes and modifications that fall within the true spirit of theinvention.

What is claimed is:
 1. An optical arrangement, comprising: at least oneplanar substrate; at least one optical waveguide planar integrated onthe at least one planar substrate; optical connecting structure; and atleast one optical waveguide structure having at least one end connectedvia the optical connecting structure to one of planar integrated opticalwaveguides, wherein the at least one optical waveguide structure ispositioned at least partly outside the integration plane for the atleast one planar integrated optical waveguide, and a refractive indexcontrast between a core region and a cladding region of the opticalwaveguide structure is at least 0.01.
 2. The optical arrangement ofclaim 1, wherein the optical waveguide structure comprises afree-standing structure which is not embedded into a liquid or a solidbody.
 3. The optical arrangement of claim 1, wherein the planarintegrated optical waveguide is a lateral single mode waveguide.
 4. Theoptical arrangement of claim 1, wherein the optical waveguide structurecomprises one of an organic material or an organically modifiedpolysiloxane compound.
 5. The optical arrangement of claim 1, whereinthe optical waveguide structure has a refractive index between the coreand cladding regions which exceeds 0.1.
 6. The optical arrangement ofclaim 1, wherein a pitch between two optical waveguide structuresmounted on the same substrate is less than 200 μm.
 7. The opticalarrangement of claim 1, wherein the optical waveguide structure has atrajectory with local curvature radii smaller than 1 mm.
 8. The opticalarrangement of claim 1, wherein the optical waveguide structure islaterally single mode.
 9. The optical arrangement of claim 1, whereinthe optical waveguide structure has each end coupled to respective onesof the planar integrated optical waveguides.
 10. The optical arrangementof claim 3, wherein the optical waveguide structure is connected betweenrespective planar integrated, lateral single-mode optical waveguides, islaterally multimode, and shaped so that in cooperation with theconnecting structures, the basic modes of the connected lateralsingle-mode optical waveguides are mapped to each other with low loss.11. The optical arrangement of claim 1, further comprising input andoutput waveguides, wherein at least one of the optical waveguidestructures has one end coupled to one of the input or output waveguidesand another end coupled to one of the planar integrated opticalwaveguides.
 12. The optical arrangement of claim 1, wherein waveguidance in the optical wave guide structure is based at least in somesections on whispering gallery modes, which propagate along a convexinterface between a high-refractive and a low-refractive material. 13.The optical arrangement of claim 1, wherein a cross-sectional shape ofthe optical waveguide structure changes along a direction of propagationfor light.
 14. The optical arrangement of claim 1, wherein the opticalwaveguide structure has a non-rotation-symmetrical cross sectionresulting in linear and uncoupled polarization states.
 15. The opticalarrangement of claim 1, wherein the planar integrated optical waveguidecomprises one of a silicon waveguide, a silicon nitride waveguide and anactive waveguide of a III-V compound semiconductor.
 16. The opticalarrangement of claim 1, wherein the optical connecting structure is partof the optical waveguide structure and comprises the same material asthe optical waveguide structure.
 17. The optical arrangement of claim 1,wherein the optical connecting structure is part of the planarintegrated optical waveguide and comprises the same material as theplanar integrated optical waveguide.
 18. The optical arrangement ofclaim 1, wherein the optical connecting structure has a shape of one ofa taper, an inverse taper, a grating coupler, or periodic structure. 19.The optical arrangement of claim 1, wherein the optical connectingstructure comprises a grating structure that couples orthogonal mainpolarization states of the optical waveguide structure into differentplanar integrated waveguides.